Recovery of metals

ABSTRACT

A method for recovering metal from electronic waste contacting electronic waste with a recovery solution to dissolve metals from the electronic waste into the recovery solution, wherein the recovery solution comprises nitric acid and ferric nitrate; and raising the pH of the recovery solution to precipitate at least some of said metals therefrom.

REFERENCE TO RELATED APPLICATIONS

The present invention relates to the recovery of valuable metals fromwaste electronics. In particular, the present invention relates to therecovery of metals such as tin from printed circuit boards. This is a USnational stage application of PCT/GB2014/052801 filed Sep. 16, 2014,claiming priority to GB 1316443.9 filed Sep. 16, 2013, the entiredisclosures of which are expressly incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the recovery of valuable metals fromwaste electronics. In particular, the present invention relates to therecovery of metals such as tin from printed circuit boards.

BACKGROUND OF THE INVENTION

In recent years, there has been concern about the growing volume ofend-of-life electronics. A large amount of electronic waste is consignedto landfill without any attempt being made to recycle the constituentmaterials. This represents both a source of pollution and a waste ofvaluable resources. With growing pressure to avoid land-filling thiselectronic waste material, there is a need to develop a method forrecycling the material and to enable recovery of the valuable metalstherein.

During the manufacture of printed circuit boards, there is commonly astep of stripping a solder mask from the board. U.S. Pat. No. 5,244,539discloses such a method for manufacturing a printed circuit board. Themethod relies upon the use of a metal-dissolving liquid to perform thestripping. The metal dissolving liquid is used to solubilise the tin andto provide a bright finish to the copper on the board. Nitric acid iscommonly used in the metal dissolving liquid. The use of variousadditives to control the extent and quality of the process are wellknown (see, for example, U.S. Pat. No. 5,505,872). In these prior artdocuments the intention is to strip a solder layer from the surface ofthe board; the methods are not conducted for sufficient time to dissolvemetals forming part of the board. Nor are these methods carried out whencomponents are present on the board, due to the risk of componentdamage.

EP 0 023 729 discloses a method for de-tinning tin plate. The method isapplied to old cans, tin plate, lacquered tin sheet waste and the like.The method uses an oxidising solution and a source of ferric ions. Themethod exploits a low temperature exchange reaction between iron andtin. There is no teaching to use this method to recover metals fromelectronic waste, nor of precipitating out dissolved metals from thede-tinning solution.

WO2013/104895 describes a method for recovering metal, such as tin, fromelectronic waste, such as printed circuit boards, using a recoverysolution comprising nitric acid and ferric chloride. Metals areseparated from the recovery solution by increasing the pH. While thismethod is capable of recovering different metals from electronic waste,approximately 30-40 wt % of the metals dissolved from the electronicwaste form a sludge. Accordingly, the yield of the method is not high.In addition, the recovered tin precipitate typically contains highlevels of iron, which decreases the value. The method is also difficultto scale up in view of the high levels of precipitates that form priorto raising the pH.

Accordingly, there is a desire for a recycling process to prevent thepresent loss of valuable metals from electronic waste into landfill.Moreover, there is a desire for a process that will overcome, or atleast mitigate, some or all of the problems associated with the methodsof the prior art or at least provide a useful or optimised alternative.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a method forrecovering metal from electronic waste, the method comprising;

contacting electronic waste with a recovery solution to dissolve metalsfrom the electronic waste into the recovery solution, wherein therecovery solution comprises nitric acid and ferric nitrate; and raisingthe pH of the recovery solution to precipitate at least some of saidmetals therefrom.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a flow chart of the method steps of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present disclosure will now be further described. In the followingpassages different aspects of the disclosure are defined in more detail.Each aspect so defined may be combined with any other aspect or aspectsunless clearly indicated to the contrary. In particular, any featureindicated as being preferred or advantageous may be combined with anyother feature or features indicated as being preferred or advantageous.

The method of the present invention is for the recovery of metals fromelectronic waste. Electronic waste, or e-waste, is a term in the artused to describe any discarded electrical or electronic devices.Electronic waste will contain useful metals from the electronicconnections and wiring required for the device to have functioned.Associated with this waste there will also be waste in the form ofcasing or wiring supports. Electronic waste can take many formsincluding, by way of example, computers, mobile phones and otherhousehold electrical devices. Electronic waste will contain a widevariety of different materials. For example, cathode ray tube monitorsand televisions may contain solder, lead, cadmium and beryllium.

Preferably, the electronic waste comprises or consists of circuit boardwaste. That is, preferably the waste includes circuit boards and/orprinted circuit boards, or broken fragments thereof. Such circuit boardwaste preferably comprises the electronic components originally solderedto a printed circuit board, since these can be readily recovered usingthe method of the present invention. A printed circuit board willnecessarily contain metal, in particular in the form of wiring andcontacts, and may further include solder applied to the contacts.

The present inventors have discovered that the use of the recoverysolution disclosed herein allows for the treatment of bulk electronicwaste. Advantageously, the use of the recovery solution comprisingnitric acid and ferric nitrate allows for the dissolution of the metalscommonly used in the electronic waste. Moreover, increasing the pHallows for the precipitation of at least some of the dissolved metalsfrom the recovery solution. These metals can then be separated, refinedand reused. In comparison to metal recovery methods known in the art,the level of sludge formation is particularly low.

The combination of nitric acid and ferric nitrate is highly oxidising.The inventors have surprisingly found that the use of such a highlyoxidising recovery solution reduces the amount of metals precipitatedfrom the recovery solution on contact with the electronic waste. Inother words, the metals remain dissolved in the recovery solution priorto increasing the pH. This enables the metals to be selectivelyrecovered from the recovery solution by selective pH precipitation, andin high yield.

This effect is particularly advantageous when the electronic wastecomprises a large number of different metals. For example, when theelectronic waste comprises silver and/or lead, the silver and leadremain in solution prior to raising the pH. This is in contrast tomethods using ferric chloride-containing or ferric sulphate-containingrecovery solutions, which typically result in precipitation of silverand lead salts on contact between the recovery solution and electronicwaste. Such precipitation may be particularly problematic when themethod is carried out on a large scale.

Compared with methods using ferric chloride or ferric sulphate recoverysolutions, the method of the present invention is capable of selectivelyrecovering tin in higher yield and with higher purity. In particular,when the electronic waste comprises tin and iron, the method of thepresent invention is capable of recovering a tin precipitate containingvery low levels of iron. This is particularly advantageous in view ofthe higher sale price of tin compared to iron. The tin precipitate mayalso contain only minimal levels of other metals, typicallysubstantially no other metals

The recovery solution may be re-used merely by the addition of nitricacid, which may serve to oxidise ferrous ions back to ferric ions. Thisresults in significant cost benefits of the method. In addition, themethod is environmentally friendly and non-complex. Due to the highlyoxidising nature of the recovery solution, more than 10 tonnes ofcircuit board waste may be processed using only one ton of recoverysolution.

The precipitated metal may be in the form of, for example, an oxide or ahydroxide. The precipitate may be recovered from the recovery solution,for example, by filtration and/or centrifuge. The pH may be raised, forexample, by the addition of NaOH.

The step of contacting the electronic waste with a recovery solutiontypically comprises immersing the electronic waste in the recoverysolution. Immersion may result in a high level of contact between therecovery solution and the electronic waste, thereby resulting in therecovery of high levels of metal from the electronic waste. Theelectronic waste is preferably immersed until substantially all of themetal in the waste is dissolved. This increases the yield of the method.The recovery solution and the immersed electronic waste may be agitated,for example by the use of ultrasonic agitation. Such agitation mayincrease the contact of the recovery solution with the electronic waste,thereby increasing the efficiency and yield of the method.

The inventors have further discovered that the recovery solution can beapplied without needing to pre-grind the electronic waste. Indeed,preferably there is no grinding step and the electronic waste is not ina powder form. It will be appreciated that the electronic waste may bedamaged or fragmented, for example, if recovered from a skip orlandfill. However, the present method allows for the treatment of suchwaste and even the separation and recovery of electronic componentstherefrom.

In one embodiment it may be advantageous to homogenise the electronicwaste with a first fragmentation step. This fragmentation steppreferably is conducted to ensure a uniform waste for treatment, withoutcompletely destroying the electronic components that may be present. Forexample, when the waste is primarily circuit board waste, the waste ispreferably fragmented to have a longest diameter of from 1 to 10 cm,more preferably from 2 to 8 cm and most preferably from 3 to 7 cm.

The present inventors have discovered that the method of the presentinvention allows for the recovery of metals from electronic componentsand also for the recovery of discreet electronic components. Thesecomponents may be separated from the waste by the dissolution of theirmetal contacts or the solder holding them to the waste, or by shearing.Moreover, because there is no need to grind the waste, it is easy tocoarsely filter and separate the treatment mixture. Due to the differentrelative sizes, without a pre-ground board it is possible to separatelyrecover (1) any non-metal waste, such as the plastic from printingcircuit boards; (2) discrete electronic components, such as transistorsor resistors; (3) any metal precipitated from the solution; and (4) therecovery solution with dissolved metals therein. Additionally, since theresidue from the above (1) is not a fine powder, it may be more readilyhandled and processed. Preferably the above materials (1) and/or (2) arerecycled as appropriate using known recycling techniques.

In one embodiment electronic components are separated from the waste byusing a standard shearing technique. One advantage of removingelectronic components in this way rather than by dissolution of theirmetal contacts or solder is that damage may be less likely to occur tothe components. For example, metals such as, for example, gold, are notlost from the components when the components are contacted with therecovery solution. Another advantage of the use of shearing is that itmay avoid an increase in the metals content (including undesirablemetals) of the recovery solution, which can lower throughput and use upnitric acid. Since the undesirable metals content is reduced, a highervalue metal cake may be obtained on precipitation. Furthermore, reducingthe metals content may serve to reduce the occurrence of a run-awayexothermic reaction that may occur at excessive metals content. In analternative embodiment, the occurrence of such a run-away exothermicreaction may be reduced by employing automated temperature controland/or cooling.

The recovery solution comprises ferric nitrate. The recovery solutionpreferably comprises from 0.5 to 12 wt % ferric nitrate, more preferablyfrom 2 to 6 wt %, even more preferably from 1 to 3.5 wt %, still evenmore preferably about 3 wt %. The ferric ions function to acceleratedissolution of tin and copper alloys. Increased levels of ferric nitrateincrease the oxidising power of the recovery solution, therebyincreasing the amount of metals that remain in solution prior to raisingthe pH. However, increased levels of ferric nitrate may result in therecovery of precipitates containing high levels of iron.

The recovery solution comprises nitric acid. The recovery solutionpreferably comprises from 10 to 60 wt % nitric acid, more preferablyfrom 15 to 40 wt %, even more preferably from 5 to 30 wt %, still evenmore preferably about 20 wt %. This concentration of the solution isparticularly effective at dissolving the metals commonly present inelectronic waste. In particular, the nitric acid functions to dissolvemetals, notably silver and tin from solder as well as copper, lead, ironand other minor metals. Any exposed gold may be liberated by underminingbut not dissolved. Gold may be recovered by further treatment stepsafter solder removal. Nitric acid may also function to re-dissolve anyferrous oxide precipitates, thereby regenerating the ferric nitrate inthe recovery solution. This may increase the lifetime of the recoverysolution. In addition, the iron content of the recovered precipitatesmay be reduced.

The recovery solution may comprise chloride ions. Chloride ions mayfunction to accelerate dissolving of the metals in the recoverysolution. In addition, the presence of chloride ions may allow allmetals to be kept in solution. Chloride ions may be in the form of, forexample, HCl, NaCl, KCl and/or NH₄Cl. However, when the electronic wastecomprises metals such as, for example, silver and lead, the amount ofchloride ions present in the recovery solution is typically kept low inorder to avoid precipitation of chloride salts such as, for example,lead chloride or silver chloride. In this regard, the recovery solutionpreferably comprises less than 5 wt % chloride ions, more preferablyless than 3 wt % chloride ions, even more preferably from 0.5 to 2 wt %chloride ions, still more preferably about 1.5 wt % chloride ions

Preferably, the recovery solution further comprises sulphamate ions.Nitric acid may be reduced by metallic species in the electronic wasteto nitrous acid. Sulphamate ions are capable of reacting with nitrousacid to convert it to sulphuric acid. Accordingly, the evolution oftoxic NO_(x) fumes and the simultaneous temperature rise can be avoided.Sulphamate ions are preferably in the form of ammonium sulphamate, whichis easy to handle and readily available. Preferably, the recoverysolution comprises from 1 to 10 wt % ammonium sulphamate, morepreferably from 2 to 6 wt %, even more preferably about 4 wt %.

Preferably, the recovery solution further comprises a copper corrosioninhibitor. Avoiding attack of copper is particularly advantageous whenthe electronic waste comprises circuit board waste. Circuit boardstypically comprise copper boards, which may be recovered from therecovery solution once the solder has been dissolved therefrom. Thecopper corrosion inhibitor is preferably an azole compound such as, forexample, benzotriazole, imidazole, pyrazole, triazole and derivativesthereof. Benzotriazole is a particularly effective copper corrosioninhibitor, is low cost, and is compatible with the other components ofthe recovery solution. In addition, benzotriazole may also form aninhibiting layer on active metals (i.e. metals that may react to form NOsuch as, for example, copper, aluminium, zinc and magnesium) present inthe electronic waste to prevent them dissolving during processing.Preferably, the recovery solution comprises up to 1 wt % benzotriazole,more preferably from 0.01 to 0.75 wt %, even more preferably from 0.01to 0.5 wt % benzotriazole, still even more preferably about 0.05 wt %.

Preferably the method further comprises subjecting the recovery solutionand electronic waste to agitation with an oxygen-containing gas,preferably air. Such agitation increases the contact between therecovery solution and the electronic waste, thereby increasing theefficiency of the method. It also serves to maintain all of thedissolved metals in a high oxidation state prior to increasing the pH.As discussed above, this is particularly important when the electronicwaste contains many different types of metal. Such agitation also servesto re-oxidise ferrous ions back to ferric ions. Accordingly, theoxidising power of the recovery solution is maintained. In addition, theiron content of the recovered precipitates may be reduced. Air agitationmay be carried out, for example, using a sparge pipe connected to an airpump.

The electronic waste is preferably immersed for at least 30 minutes inthe recovery solution, more preferably from 30 minutes to 24 hours, evenmore preferably from 2 to 12 hours. Longer treatments may be suitable,but process efficiency favours a faster processing time. Lower gradeboards may typically be processed in from 2 to 4 hours, more typicallyabout 3 hours. Higher grade boards may typically be processed in from 5to 20 hours, more typically about 8 hours. Preferably the treatment isconducted in a batch-wise manner.

Preferably, the method further comprises adding chloride ions to therecovery solution prior to raising the pH. This may allow theprecipitation and recovery of metals such as silver and lead in the formof chlorides. It is preferable to add the chloride ions to the recoverysolution in a separate step rather than by incorporating large amountsof chloride ions into the recovery solution from the start, so thatprecipitation of metals such as silver and lead may be carried out in acontrolled manner.

The pH is preferably raised to from 3 to 6, more preferably from 3 to 5,even more preferably from 3.8 to 4. When the electronic waste comprisestin, such pH values may result in precipitation of a tin oxide-richprecipitate, from which tin may be recovered. In view of the highlyoxidising nature of the recovery solution, when iron is present in theelectronic waste, the iron tends to remain in solution at such pHvalues. Accordingly, the amount of iron present in the tin oxide-richprecipitate is low.

In a preferred embodiment, the method further comprises:

-   -   (i) raising the pH to from 3 to 6;    -   (ii) raising the pH to greater than 6; and    -   (iii) lowering the pH to from 3 to 6,

wherein precipitate formed by steps (i), (ii) and (iii) is recoveredfrom the recovery solution before the next step.

Such a method is particularly advantageous when the electronic wastecomprises both iron and tin. As discussed above, step (i) may result ina tin oxide-rich precipitate containing only low levels of iron. In step(ii), raising the pH to 6, preferably from 6.5 to 7.5, more preferablyto about 7, may result in the precipitation of an iron-rich precipitate.Accordingly, lowering the pH in step (iii) to from 3 to 6, preferablyfrom 3 to 5, more preferably from 3.8 to 4, may result in a tin-oxiderich precipitate which contains very low levels of iron, typicallysubstantially no iron. This is particularly beneficial in view of thehigher price of tin relative to iron. Furthermore, reducing the amountof iron in the precipitate enables the precipitate to be dried to agreater extent, typically to contain less than 5wt % water, moretypically about 3 wt % water. This may be advantageous with regard totransportation and/or processing of the precipitate.

Preferably, contacting the electronic waste with the recovery solutionis carried out in a revolving drum. The use of a revolving drumincreases the contact between the electronic waste and the recoverysolution, and may ensure that the recovery solution comes into contactwith all parts of the electronic waste. Accordingly, the efficiency andyield of the method is increased. Furthermore, when the electronic wastecomprises circuit board waste, the use of a revolving drum may help toknock off components from the board, which may be recovered from therecovery solution for recycling or reuse.

Preferably, the method further comprises continuous filtration of therecovery solution. This may help to keep the solution clean and free ofdeleterious material during the method. In addition, when combined withair agitation, continuous filtration may help to keep the dissolvedmetals in a high oxidation state. Continuous filtration may be carriedout, for example, using a membrane press.

The method may be a batch process or a continuous process. Preferably,the method is a batch process and the recovery solution is regeneratedwith nitric acid between batches. As discussed above, nitric acid mayre-oxidise ferrous ions in the recovery solution back to ferric ions.

The electronic waste preferably comprises circuit-board waste. Themethod is particularly effective at recovering metals from circuit-boardwaste. This is because, inter alia, it is particularly effective atrecovering the metals typically contained in circuit boards, inparticular circuit board solders, for example solders containing tin,lead and silver. In addition, circuit board waste typically comprisesmultiple types of metal.

The electronic waste is preferably contacted with a recovery solutionhaving a pH of less than 1. More preferably, the pH of the solution willbe from −1 to 1 and most preferably about 0. The use of these harshconditions allows for the full dissolution of metals from the electronicwaste into the recovery solution.

Preferably, the method further comprises removing any electronic wasteresidue from the recovery solution before raising the pH. This may allowthe precipitated metals to be recovered from the recovery solution moreeasily.

The electronic waste is typically not provided in the form of a powder.

The step of raising the pH is preferably conducted stepwise to allow forthe separate precipitation and recovery of different metals. Thisprovides a cost effective method of separating out the different metalsrecovered.

The present inventors have discovered that the method is suitable forthe recovery of one or more of lead, gold, silver, copper, zinc and tinfrom electronic waste. Obviously, the metals that can be recovered willdepend upon the electronic waste and the metals contained therein.Preferably the method is used for at least the recovery of tin fromelectronic waste. Tin is a common component in solders and will commonlybe present in electronic waste. Preferably the method is used forrecovery of metals from electronic waste containing lead-free solders.The method is particularly suitable for the recovery of tin, copper andsilver, which are typical components of lead free solders. Preferablythe method further comprises a step of subjecting the precipitatedmetals to a conventional recovery treatment to obtain one or moresubstantially pure metals therefrom.

The process of the present invention may be carried out by a standaloneunit. Alternatively, the process may be carried out by an “add-on” unitto a standard waste PCB processing unit.

The method may further comprise recovering the precipitated metals fromthe recovery solution, preferably by filtration or centrifugation. Therecovered precipitated metals may be dried, preferably using a rotarydrying oven. Such drying enhances the metal concentration in the finalmetal cake, which may result in a more valuable product. The use of arotary drying oven may result in the final metal cake comprising lessthan 5 wt % water, more typically about 3 wt % water.

The method may further comprise subjecting the precipitated metals tofurther treatment to recover one or more substantially pure metals. Thefurther treatment may comprise, for example, smelting and/orelectrowinning.

The method may comprise additionally recovering and sorting electroniccomponents separated from the electronic waste by the dissolution ofmetal from the electronic waste. Such electrical components may bere-used or recycled as appropriate using known techniques.

In a preferred embodiment of the method:

the metals recovered comprise tin;

the recovery solution and electronic waste are subjected to airagitation;

the pH is raised to from 3 to 6; and

the recovery solution comprises:

-   -   i. from 1 to 3.5 wt % ferric nitrate;    -   ii. from 15 to 40 wt % nitric acid;    -   iii. from 0.5 to 2 wt % chloride ions;    -   iv. from 1 to 10 wt % ammonium sulphamate; and    -   v. from 0.01 to 0.5 wt % benzotriazole.

The invention will now be discussed further with reference to theFigure, provided purely by way of example, in which:

FIG. 1 shows a flow chart of the method steps of the present invention.The reference numerals refer to the following: (1) electronic waste, (2)recovery solution, (3) immersion, (4) agitation, (5) separation, (6) anyinitial precipitate, (7) waste residue (plastics), (8) electroniccomponents, (9) recovery solution and dissolved metals, (10) increasepH, (11) precipitated metals, and (12) spent recovery solution.

EXAMPLES

The invention will be described with reference to the following exampleswhich are provided by way of example and are not limiting.

Example 1

The following aqueous recovery solution was prepared (in weight %):

20% nitric acid

1.5% hydrochloric acid

6% ferric nitrate

4% ammonium sulphamate

0.1% benzotriazole

4.285 Kg of mixed circuit boards were stripped of electronic componentsbefore being immersed in 500 ml of the recovery solution in a reactorvessel. The solution was operated in static mode using continuousfiltration and employed air agitation when the solution darkened. Thesolution was occasionally replenished by adding 40 ml nitric acid and 5g ferric nitrate. The final volume of recovery solution was 900 ml.

After approximately 3 hours, the recovery solution was treated with HClto precipitate lead and some silver from the recovery solution asinsoluble chlorides. The pH was then increased step-wise to selectivelyrecover metals dissolved in the recovery solution. The compositions ofthe precipitates recovered at each stage are set out below in Table 1.

TABLE 1 Chemical compositions of precipitates. % Cu % Pb % Zn % Fe % SnAg (mg/kg) Precipitate formed when 0.094 72.71 0.006 0.45 0.36 158.0 900ml solution treated with HCl After lead removal, solution divided into500 ml sample and a 400 ml sample: 500 ml taken to pH 7 0.032 1.86 0.238.39 39.85 105.2 Results: dry ppt 400 ml taken to about pH 0.047 2.670.13 1.81 53.28 163.2 4 to recover predominately tin ppt After tinremoval, 0.15 12.63 1.00 46.52 0.36 47.90 resultant solution taken to pH7 to give ppt Wash water treated at 0.20 4.38 0.018 0.11 60.60 1954about pH 4 Cu mg/l Pb mg/l Zn mg/l Fe mg/l Sn mg/l Ag mg/l Final washwater after all 3.04 0.05 0.54 0.10 0.14 0.03 metals removed

Example 2

The following aqueous recovery solution was prepared (in weight %):

20% nitric acid

1.5% hydrochloric acid

3% ferric nitrate

4% ammonium sulphamate

0.1% benzotriazole

3.555 Kg of mixed circuit boards were stripped of electronic componentsbefore being immersed in 500 ml of the recovery solution in a reactorvessel. The solution was operated in static mode using continuousfiltration. The solution was occasionally replenished by adding 40 ml of50% v/v nitric acid.

After approximately 3 hours, the recovery solution was treated with HClto precipitate lead and some silver from the recovery solution asinsoluble chlorides. The pH was then increased step-wise to selectivelyrecover metals dissolved in the recovery solution. The compositions ofthe precipitates recovered at each stage are set out below in Table 2.

TABLE 2 Chemical compositions of precipitates. % Cu % Pb % Zn % Fe % SnAg (mg/kg) Precipitate formed when 0.001 78.69 0.003 0.005 0.53 60.55solution treated with HCl Precipitate formed when 0.024 7.01 0.30 3.2146.69 83.59 solution taken to pH 7 Wash water pH 7 0.59 21.70 0.40 2.8841.90 2879 precipitate Cu mg/l Pb mg/l Zn mg/l Fe mg/l Sn mg/l Ag mg/lFinal pH 7 solution after 0.84 0.36 0.07 0.36 0.35 0.01 metals removed

The results demonstrate that the efficiency of the solution was noteffected by reducing the ferric nitrate content to 3%. Furthermore, thesolution only needed to be replenished by adding nitric acid.

It was also found that the solution could be treated to about pH 5 torecover a higher tin concentrate prior to pH 7 treatment to remove iron.

The foregoing detailed description has been provided by way ofexplanation and illustration, and is not intended to limit the scope ofthe appended claims. Many variations in the presently preferredembodiments illustrated herein will be apparent to one of ordinary skillin the art and remain within the scope of the appended claims and theirequivalents.

1. A method for recovering metal from electronic waste, the methodcomprising; contacting electronic waste with a recovery solution todissolve metals from the electronic waste into the recovery solution,wherein the recovery solution comprises nitric acid and ferric nitrate;and raising the pH of the recovery solution to precipitate at least someof said metals therefrom.
 2. The method of claim 1, wherein the recoverysolution comprises from 0.5 to 12 wt % ferric nitrate.
 3. The method ofclaim 1, wherein the recovery solution comprises from 10 to 60 wt %nitric acid, preferably from 15 to 40 wt %.
 4. The method of claim 1,wherein the recovery solution comprises chloride ions.
 5. The method ofclaim 1, wherein the recovery solution further comprises sulphamateions.
 6. The method of claim 1, wherein the recovery solution furthercomprises a copper corrosion inhibitor.
 7. The method of claim 1,further comprising subjecting the recovery solution and electronic wasteto agitation with an oxygen-containing gas.
 8. The method according toclaim 1, wherein the electronic waste is immersed for at least 30minutes in the recovery solution.
 9. The method of claim 1, furthercomprising adding chloride ions to the recovery solution prior toraising the pH.
 10. The method of claim 1, wherein the raising the pH ofthe recovery solution comprises raising the pH to from 3 to
 6. 11. Themethod of claim 1, further comprising: (i) raising the pH to from 3 to6; (ii) raising the pH to greater than 6; and (iii) lowering the pH tofrom 3 to 6, wherein precipitate formed by steps (i), (ii) and (iii) isrecovered from the recovery solution before the next step.
 12. Themethod of claim 1, wherein contacting the electronic waste with therecovery solution is carried out in a revolving drum.
 13. The method ofclaim 1, further comprising continuous filtration of the recoverysolution.
 14. The method of claim 1, wherein method is a batch processand the recovery solution is regenerated with nitric acid betweenbatches.
 15. The method according to claim 1, wherein the electronicwaste comprises circuit-board waste.
 16. The method according to anyclaim 1, wherein the electronic waste is contacted with a recoverysolution having a pH of less than
 1. 17. The method according to claim1, the method further comprising removing any electronic waste residuefrom the recovery solution before raising the pH.
 18. The methodaccording to claim 1, wherein the electronic waste is not provided inthe form of a powder.
 19. The method according to claim 1, wherein thestep of raising the pH is conducted stepwise to allow for the separateprecipitation and recovery of different metals.
 20. The method accordingto claim 1, wherein the metals recovered comprise one or more of lead,silver, iron, copper, gold, zinc and tin.
 21. The method of claim 1,further comprising recovering the precipitated metals from the recoverysolution.
 22. The method according to claim 1, the method furthercomprising subjecting the precipitated metals to further treatment torecover one or more substantially pure metals.
 23. The method accordingto claim 1, wherein the method comprises additionally recovering andsorting electronic components separated from the electronic waste by thedissolution of metal from the electronic waste.
 24. The method of claim1, wherein: the metals recovered comprise tin; the recovery solution andelectronic waste are subjected to air agitation; the pH is raised tofrom 3 to 6; and the recovery solution comprises: from 1 to 3.5 wt %ferric nitrate; from 15 to 40 wt % nitric acid; from 0.5 to 2 wt %chloride ions; from 1 to 10 wt % ammonium sulphamate; and from 0.01 to0.5 wt % benzotriazole.